F-18 FLUORODEOXYGLUCOSE POSITRON EMISSION TOMOGRAPHY IN THE FOLLOW-UPOF DIFFERENTIATED THYROID-CANCER

Whole-body fluorine-18 fluorodeoxyglucose (FDG) positron emission tomo graphy (PET) imaging was performed during the follow-up of 33 patients suffering from differentiated thyroid cancer. Among them there were 2 6 patients with papillary and seven with follicular tumours. Primary t umour stage (pT) was pT1 in six cases, pT2 in eight cases, pT3 in thre e cases and pT4 in 14 cases. FDG PET was normal in 18 patients. In thr ee patients a slightly increased metabolism was observed in the thyroi d bed, assumed to be related to remnant tissue. In one case local recu rrence, in ten cases lymph node metastases (one false-positive, caused by sarcoidosis) and in three cases distant metastases were found with FDG PET. In comparison with whole-body scintigraphy using iodine-131 (WBS) there were a lot of discrepancies in imaging results, Whereas th ree patients had distant metastases (proven with I-131) and a negative FDG PET, in four cases I-131-negative lymph node metastases were dete ctable with PET. Even in the patients with concordant ''staging'', dif ferences between I-131 and FDG were observed as to the exact lesion lo calization. Therefore, a coexistence of I-131-positive/FDG-negative, I -131-negative/FDG-positive and I-131-positive/FDG-positive malignant t issue can be assumed in these patients. A higher correlation of FDG PE T was observed with hexakis (2-methoxyisobutylisonitrile) technetium-9 9m (I) (MIBI) scintigraphy (performed in 20 cases) than with WBS, In h ighly differentiated tumours I-131 scintigraphy had a high sensitivity , whereas in poorly differentiated carcinomas FDG PET was superior. Th e clinical use of FDG PET can be recommended in all cases of suspected or proven recurrence and/or metastases of differentiated thyroid canc er and is particularly usefull in cases with elevated serum thyroglobu lin levels and negative WBS.